A solar collector may receive solar radiation (i.e., sunlight) and direct the solar radiation onto a photovoltaic (or, solar) cell. A “concentrating” solar collector may also convert the received solar radiation into a concentrated radiation beam prior to directing the radiation onto the solar cell. The cell, in turn, may generate electrical power based on photons of the received radiation.
A solar collector is designed to generate power in response to radiation which intercepts the solar collector within a certain range of incidence angles. Power generation typically drops significantly if incoming radiation deviates from the range of incidence angles. The range depends on the design of the solar collector, and typically narrows with increasing concentration factors. For example, in some solar collector designs providing approximately 500-fold concentration, the range of incidence angles providing suitable power generation extends only one degree from normal.
In operation, a solar collector is aligned with a radiation source (e.g., the sun) such that incoming radiation intercepts the solar collector within its preferred range of incidence angles. According to some systems, a solar collector is associated with a central axis perpendicular to its reception surface, and the foregoing alignment consists of moving the solar collector so that the axis points directly toward the apparent position of the sun in the sky. As mentioned above, power generation may be significantly compromised due to any errors in aligning the axis with the sun. Possible causes of such error include errors in determining the sun's apparent position, and mechanical calibration errors, which may result in misalignment between the axis and the sun even if the sun's position is known.
Various control systems for aligning a solar collector with the sun are known. In a closed loop control system, a sun sensor is aligned to an axis representing a preferred incidence angle of the solar collector. The sun sensor determines any error between the axis and the sun's apparent position, and a position of the solar collector is changed to compensate for the error. Closed loop control systems may incorrectly determine (or may be unable to determine) the solar position if the sun is obscured (e.g., by clouds or debris deposited on the sun sensor) or if a portion of the solar collector is obscured. Closed loop control systems are also required to maintain the alignment between the sun sensor and the axis while being subjected to environmental hazards.
An open loop control system may determine the position of the sun based on a location of the solar collector, a current time, and a set of ephemeris equations. The solar collector is positioned so that the determined solar position results in reception of radiation at its preferred angle of incidence. Since such open loop control systems do not provide any feedback for determining whether the solar collector is correctly positioned (i.e., aligned with the sun and/or generating maximum power for a given solar intensity), these systems are susceptible to mechanical errors and perturbations.
Many factors may prevent an open loop control system from aligning a solar collector with a properly-calculated solar position. These factors include manufacturing tolerances, wind and gravity loading, subsidence of a solar collector foundation, creep in the mounting of the collector or its internal optics, and optic degradation. Some systems address these and other factors by determining correction information and using the correction information to correct a calculated solar position or to correct alignment of a solar collector with a given solar position.
The above-described correction information may be determined every few weeks or months. To determine correction information, a solar collector may be aligned with various positions until a maximum short-circuit current (Isc) is detected. Correction factors are determined based on a difference between a calculated determined solar position and a position at which the maximum Isc was detected. Some systems collect such correction factors at several different times of day. The correction factors may be weighted against previously-acquired correction factors and/or interpolated to determine correction factors to be used at times of day other than those at which correction factors was collected.
A solar collector does not deliver power during the above-described collection of correction factors. Moreover, new alignment errors may occur during the weeks or months between correction factors collection. Accordingly, the foregoing systems are unable to correct for such new errors until a next collection of correction factors. Even if no new errors have occurred since a last collection of correction factors, the correction factors will only provide a solar collector position at which Isc is maximized. In contrast, it would be more desirable to provide a system which attempts to maximize actual power delivered to the grid.